1.

Introduction

High definition (HD) video services have become widely available in recent years and demands for high quality video services have also been rapidly increasing. Moreover, it is expected that the storage space and transmission bandwidth capacity will lead to further increases in the production, storage, and delivery of high quality video services. For example, uncompressed HD video signals require about 1 Gbps and uncompressed ultra high definition (UHD) video signals require about 4 Gbps (UHD-4k, $3840\times 2160$) or 15 Gbps (UHD-8k, $7680\times 4320$). Therefore, video compression technology is essential for high quality video services. Due to this, a number of international standards have been established, such as Moving Picture Experts Group (MPEG)-2, MPEG-4, H.263, and H.264. Recently, the MPEG and the international telecommunication union telecommunication standardization sector (ITU-T) video coding experts group have been jointly developed for high efficiency video coding (HEVC) standards.

In conventional video compression methods, the goal is to minimize the mean squared error (MSE) or sum of absolute difference (SAD) metrics. These metrics have typically been used for rate-distortion (RD) optimization. However, it has been reported that MSE and SAD metrics do not accurately represent perceptual quality. Therefore, MSE has sometimes been replaced with other metrics that better reflect perceptual images or video quality.¹ For example, the just noticeable distortion (JND) model estimation was implemented on a H.264/AVC system to optimize the spatial–temporal human visual systems.² Also, in recent coding standard activities, perceptual evaluation has also been used together with peak signal-to-noise ratios (PSNRs).

There have been several attempts to measure the relationship between perceptual quality and spatial frequency. Since the discrete cosine transform (DCT) method has been widely used in most video compression standards such as H.261, H.263, H.264, MPEG1, MPEG2, and MPEG4, JND models based on the DCT domain have been studied. In Ref. 3, distortion visibility thresholds for DCT coefficients were approximated using luminance-based models. The DCTune model uses luminance adaptation and contrast masking effects to optimize the JPEG DCT quantization matrix.^4,5 A block classification based DCTune was proposed to improve the perceptual image coding performance in Ref. 6. A DCT-based JND model for monochrome image/video using contrast sensitivity functions was proposed in Ref. 7. In Ref. 2, quantization steps for low frequencies were allocated smaller values than for higher frequencies. These findings about frequency sensitivity have mainly focused on image coding and have used $8\times 8$ float DCT coefficients.

There has been a research in spatial frequency sensitivity.^8–10 Based on subjective experiments where specially designed patterns were used, the contrast sensitivity function of spatial frequency was defined as the sensitivity level according to the spatial frequency.⁸ All the previous experiments were performed using simulated one-dimensional (1-D) signals. However, video coding usually deals with two-dimensional (2-D) frequency sensitivities. Thus, those previous research results may not be directly applicable to image or video coding. Also, most coding methods use block transforms such as DCT, where each coefficient represents a 2-D frequency component. Our preliminary experiments showed that errors in middle frequencies did not cause less severe perceptual degradation compared to errors in lower and higher frequencies. This observation will be discussed in greater detail in Sec. 3.

In this paper, we investigate the frequency sensitivity of the human visual system in video coding through extensive subjective testing. We observed that the human visual system reacts differently at different frequencies. Therefore, we used different quantization steps for different frequency components. In other words, small quantization steps were used for sensitive frequency components while large quantization steps were used for less sensitive frequency components. Joint model (JM), the reference encoder of the H.264/AVC standard, was used for test frequency sensitivity.

2.

Quantization of DCT Coefficients

The DCT is widely used in numerous applications in image and video lossy compression technologies. For example, DCT is used in video compression standards such as MPEG-2, MPEG-4, H.261, H.263, H.264, and HEVC. DCT helps to separate images into spectral subbands of differing importance. Lower frequency components are more important than higher frequency components for video quality. Moreover, in most video data, low frequency components are dominant. The forward and inverse DCT values can be defined as follows:

Using this definition, the $4\times 4$ DCT can be specified as follows:

Figure 1 shows the basis functions of the $4\times 4$ DCT. In $N\times N$ DCT applications, there is one DC component and $N\times N-1$ AC components. The energy of the DC component is dominant in most cases. Typically, the energies of the AC components are smaller compared with the DC components. This energy compaction property has been exploited in compression methods along with the quantization technique.

Fig. 1

$4\times 4$ DCT basis patterns.

The quantization process is essential to compress video data. Quantized coefficients are usually computed as follows:

Table 1

Quantization step size according to the QP value used in the H.264 standard.

In some image and video coding standards, an optional quantization technique using a quantization matrix (q-matrix) is provided. In this basic quantization method, the same quantization step is adapted to all DCT coefficients. The q-matrix provides a full matrix for the quantization modification coefficients. Different quantization steps for different DCT coefficients can be used with the q-matrix. Figure 2 shows how the q-matrix can be used for $4\times 4$ DCT coefficients. The q-matrix is inserted in the compressed bit stream header. This matrix should be designed to achieve maximum perceptual quality with high compression efficiency.

Fig. 2

Quantization with q-matrix.

3.

Frequency Modeling for DCT

The 2-D DCT coefficients were represented as a 1-D vector using the zigzag scanning method. Figure 3 shows the 1-D frequency representation of the 2-D DCT coefficients.

Fig. 3

One-dimensional (1-D) frequency representation with zigzag scanning.

To examine the amount of human frequency sensitivity on video coding, several quantization methods were designed and tested in the experiments. We performed a number of subjective tests to evaluate the perceptual quality of various frequency quantization models. In our subjective tests, we used three or four different models per session.

Previous research on spatial frequency sensitivity has concluded that middle range frequencies are more sensitive to human perception than low and high range frequencies.⁸ However, this research used simple sinusoidal patterns (Fig. 4) to investigate spatial frequency sensitivity and is not always directly applicable to video coding in real-world situations. Figure 5 shows three images with the same PSNR with degradations in different frequency ranges. It can be seen that the image with degradations in the middle frequencies [Fig. 5(b)] shows better perceptual quality than the images with degradations in the low or high frequencies [Figs. 5(a) and 5(c)]. Based on this observation, we used larger quantization steps for middle frequency coefficients.

Fig. 4

Spatial frequency pattern used in Ref. 8.

Fig. 5

Degraded Lena images with the same PSNR (29.51 dB) (a) low frequency degradations, (b) middle frequency degradations, (c) high frequency degradations, (d) difference image of low frequency degradations, (e) difference image of middle frequency degradations, and (f) difference image of high frequency degradations.

In the first set, four different frequency quantization methods were designed as shown in Fig. 4. In the proposed methods, the quantization multiplier was used to adjust the quantization step as follows:

Consequently, a large value of the quantization multiplier resulted in a large quantization step, which produced smaller compressed data at low-image quality. If the value of the quantization multiplier was 1, the original quantization step was used.

In Fig. 6(a), a trapezoid multiplier function is shown. In this model, the middle frequency components were more coarsely quantized than the lower or higher frequency components. In Fig. 6(b), a triangle multiplier function is shown. In the triangle function (triangle mode 1), the middle frequency components were also more coarsely quantized similar to the trapezoid function with a peak point. Figure 6(c) shows a linearly increasing function and Fig. 6(d) shows a linearly decreasing function. These four frequency quantization multiplier functions were calculated as follows:

Fig. 6

Quantization multiplier functions (set 1) for H.264/AVC $4\times 4$ DCT coefficients: (a) trapezoid, (b) triangle mode 1, (c) linearly increasing, and (d) linearly decreasing.

The second set (set 2) of multiplier functions is shown in Fig. 7. In this set, various shapes for middle frequencies were tested. The triangle mode1 shown in Fig. 7(a) was the same as the triangle function in Fig. 6(b) of set 1. Also, the linear increasing function shown in Fig. 7(d) was also the same as the linearly increasing function of set 1. The triangle mode 2 function shown in Fig. 7(b) was a combined model of the triangle mode 1 and the linear increasing functions. In other words, in ascending parts (low frequencies), the function used the linearly increasing function in Fig. 7(d) while the triangle mode 1 function was used in descending parts (high frequencies). Figure 7(c) shows another modified version of the triangle function. This triangle mode 3 preserved the high frequency components. The two new frequency quantization functions were calculated as follows:

Fig. 7

Quantization multiplier functions (set 2) for H.264/AVC $4\times 4$ DCT coefficients: (a) triangle mode 1, (b) triangle mode 2, (c) triangle mode 3, and (d) linearly increasing.

Figure 8 shows the third set of frequency quantization functions (set 3) using three different triangle functions with different peak values. Figure 8(b) was the same as the triangle mode 1 function. These three frequency quantization multiplier functions were calculated as follows:

Fig. 8

Quantization multiplier functions (set 3) for H.264/AVC $4\times 4$ DCT coefficients: (a) triangle (low), (b) triangle (med), and (c) triangle (high).

Figure 9 shows the fourth set (set 4) of the frequency quantization functions, which includes two additional functions with coarse quantization for the middle frequencies. The triangle and trapezoid functions were identical to those of Fig. 6. The two new frequency quantization functions were calculated as follows:

Fig. 9

Quantization multiplier functions (set 4) for H.264/AVC $4\times 4$ DCT coefficients: (a) triangle, (b) cosine, (c) quadratic, and (d) trapezoid.

A total of nine quantization multiplier functions were designed. Since a large value of the quantization multiplier function produced a large quantization step, the area of the quantization multiplier function was related to the average quantization step. Table 2 shows the areas of the nine multiplier functions. The triangle (high) quantization showed the largest area while the triangle mode 3 quantization showed the smallest area. The linearly increasing quantization had the same area as the linearly decreasing quantization. However, the linearly decreasing function produced smaller compressed data than the linearly increasing function since the energy of the low frequency components was dominant.

Table 2

Areas of the quantization multiplier functions.

4.

Subjective Assessments

Subjective quality assessment was performed to investigate the frequency sensitivity of the human visual system. Six subjective tests were conducted using the frequency quantization sets. In each subjective test, four QPs were selected, which reflected various levels of coding quality. Table 3 shows the test designs. In each test design, three different conditions were considered: source video sequences, QPs, and quantization methods.

Table 3

Subjective test designs for various frequency quantization sets.

In the HD test 1 experiment, nine source video sequences, four QPs, and five quantization methods were used. The nine source video sequences of full HD ($1920\times 1080$) were selected based on compression difficulties. Each source video sequence was 8-s long with 30 fps (240 frames). The default setting of H.264/AVC (JM 15.1) was used for the original quantization method. The frequency quantization set 1 was used in this design along with the original quantization method (default setting). In the test, 189 processed video sequences (PVS) were generated according to the experimental design. The number of PVSs was calculated as follows:

In the subjective test of HD test 2, the same source video sequences of HD test 1 were used. Four QPs were used in this test. In this test, denser QPs were used than the QPs of HD test 1. In HD test 2, the difference between adjacent QPs was 3 while the difference was 5 in HD test set 1. The frequency quantization set 2 was used in HD test 2 along with the original quantization method. In this design, 189 PVSs were generated.

In the subjective test of video graphics array (VGA) test 1, the frequency quantization set 1 was used with VGA ($640\times 480$) source videos. Each source video sequence was 12-s long with 30 fps (360 frames). Four quantization methods are selected in quantization set 1. The QPs, frame rates, and length of video clips were the same as those of HD subjective test 1. However, different resolution and source contents were used. In the subjective test of VGA test 2, the frequency quantization set 2 was used with VGA ($640\times 480$) source video sequences. Four quantization methods were selected in the quantization set 2. In the subjective test of VGA test 3, the frequency quantization set 3 was used with VGA ($640\times 480$) source video sequences. Nine source video sequences, four QPs and four quantization methods were used in this subjective test. In this test, 153 PVSs were generated. In the subjective test of VGA test 4, the frequency quantization set 4 was used with VGA ($640\times 480$) source video sequences. Seven source videos, four QPs, and five quantization methods were used in VGA test 4. In this design, 147 PVSs were generated.

The viewing environments were set in accordance with ITU-T and ITU-R standards.^12,13 Lighting and display characteristics were tuned according to the standard specifications. Figure 10 shows the viewing distance setting in the subjective quality assessment in the HD tests. The distance between the display monitor and the viewers was set to $3H$, where $H$ represents the height of the display monitor. Two viewers watched the video sequences at the same time in the HD tests.

Fig. 10

Viewing distance in terms of subjective quality assessment.

To evaluate video subjective quality, the absolute category rating–hidden reference (ACR–HR) method was used. Figure 11 shows an example of the viewing order of the ACR–HR method. In this method, each video was played once in a random order. Also, reference video sequences were hidden in the video clips. Viewers did not know which video sequence was a reference video sequence. Between the video sequences, gray videos were inserted. When gray videos were played, viewers rated the video quality.

Fig. 11

The viewing order and viewing time of the ACR-HR method.

In the quality ratings, every video sequence was rated in terms of five categories as shown in Fig. 12: excellent, good, fair, poor, and bad. Results were converted into numerical scores on a 1 to 5 scale. A single score for one video sequence was calculated by averaging all the numerical scores of 24 viewers. Then, the difference mean opinion scores (DMOS) were calculated as follows:

Fig. 12

Quality grades used in the ACR-HR method.

5.

Experimental Results

Figure 13 shows the experimental results of the bitrates and the subjective quality rating of the frequency quantization set 1 with HD clips. In this experiment, the linearly decreasing model produced the lowest bitrates among the four quantization methods as shown in Fig. 13(d), while the linearly increasing model produced the highest bitrates as shown in Fig. 13(c). However, the linearly increasing model showed the best subjective quality while the linearly decreasing model showed the worst subjective quality. Since the low frequency components in the DCT domain had higher energy levels than the high frequency components, a large bitrate reduction of the linearly decreasing model was expected. The subjective scores (DMOS) were generally proportional to the bitrate reduction ratio. Figure 14 shows a performance comparison in terms of the subjective scores (DMOS), PSNR, and SSIM. The Structural SIMilarity (SSIM) measures structural similarity between two images. It is known that SSIM is better correlated with the human visual system than PSNR.¹⁴ It appears that all the frequency quantization functions of set 1 produced subjective scores that were better than those of the reference method as shown Fig. 14(a). Except for the linearly decreasing model, the frequency quantization functions showed similar performance compared to the reference model in terms of PSNR and SSIM.

Fig. 13

Bitrate and subjective score comparison of the frequency quantization set 1 (HD): (a) trapezoid, (b) triangle, (c) linearly increasing, and (d) linearly decreasing.

Fig. 14

Performance comparison for the frequency quantization set 1 (HD): (a) subjective scores, (b) PSNR, and (c) SSIM.

To investigate the coding efficiency, the bitrates of the quantization functions, which produced equivalent perceptual quality of the reference method, were compared with those of the reference method. For instance, if a proposed quantization model had a $-50\%$ bitrate reduction ratio, only 50% of the bitrates could produce subjective quality equivalent to that of the reference method. Table 4 shows the results of bitrate reduction. Although the linearly increasing model showed the worst bitrate reduction, it showed the best efficiency improvement among the four models in terms of perceptual quality.

Table 4

Bitrate reduction ratio for each quantization set.

Figure 15 shows the experimental results of the bitrates and the subjective quality rating of the frequency quantization set 1 with VGA clips. The linearly increasing model [Fig. 15(a)] and triangle model [Fig. 15(c)] showed minor subjective quality degradations while the linearly decreasing model [Fig. 15(b)] showed large subjective quality degradations. Also, the linearly decreasing function also produced larger bitrate reductions that it did in the HD test [Fig. 13(d)]. Obviously, applying a large quantization step to low frequency components resulted in a large bitrate reduction and a large perceptual quality degradation. Table 4 shows the bitrate reduction ratios, which produced the perceptual quality equivalent to the reference method. The linearly increasing model showed the best bitrate reduction while the linearly decreasing model showed poor performance, requiring more bits to produce the equivalent perceptual quality. Figure 16 shows a performance comparison in terms of the subjective scores, PSNR, and SSIM. The linearly increasing and triangle models showed slightly improved performance in terms of DMOS and SSIM when compared to the reference model while the linearly decreasing model showed inferior performance.

Fig. 15

Bitrate and subjective score comparison of the frequency quantization set 1 (VGA): (a) linearly increasing, (b) linearly decreasing, and (c) triangle.

Fig. 16

Performance comparison for the frequency quantization set 1 (VGA): (a) subjective scores, (b) PSNR, and (c) SSIM.

Figure 17 shows the experimental results of the bitrates and the subjective quality rating of the frequency quantization set 2 (Fig. 7) with HD clips. In this experiment, the triangle mode 1 function produced the lowest bitrates among the four functions, as shown in Fig. 17(a), while the triangle mode 3 function produced the highest bitrates, as shown in Fig. 17(c). The triangle mode 3 function showed the worst bitrate reduction and produced inconsistent subjective scores. Table 4 showed the bitrate reduction ratios that produced the same subjective quality as that of the reference model for the quantization set 2. The triangle mode 3 function showed the best bitrate reduction among the four models while the linearly increasing function showed poor performance, requiring more bits to produce equivalent perceptual quality. Figure 18 shows a performance comparison in terms of the subjective scores, PSNR, and SSIM.

Fig. 17

Bitrate and subjective score comparison for the frequency quantization set 2 (HD): (a) triangle mode 1, (b) triangle mode 2, (c) triangle mode 3, and (d) linearly increasing.

Fig. 18

Performance comparison for the frequency quantization set 2 (HD): (a) subjective scores, (b) PSNR, and (c) SSIM.

Figure 19 shows the experimental results of the bitrates and the subjective quality of the frequency quantization set 2 with VGA source sequences. In this experiment, all the quantization models showed large subjective score degradations as shown in Figs. 19(a)–19(c). Although the three quantization functions achieved large bitrate reductions, it appears that the subjective score degradations were larger. Consequently, the overall coding efficiency considering the bitrate reduction and the subjective scores appeared to decrease as shown in Fig. 20 and Table 4. The triangle functions showed inconsistent performance and their usefulness was rather limited.

Fig. 19

Bitrate and subjective score comparison for the frequency quantization set 2 (VGA): (a) triangle mode 1, (b) triangle mode 2, and (c) triangle mode 3.

Fig. 20

Performance comparison for the frequency quantization set 2 (VGA): (a) subjective score, (b) PSNR, and (c) SSIM.

Figure 21 shows the experimental results of the bitrates and the subjective quality of the frequency quantization set 3 with VGA source sequences. In the frequency quantization set 3, three different triangle functions with different quantization intensities were used. The triangle-low function [Fig. 21(a)] had the smallest peak value while the triangle-high function [Fig. 21(c)] had the largest peak value. Generally, subjective scores and bitrate reductions are proportional to the peak values. Table 4 shows coding efficiency comparisons. Figure 22 shows a performance comparison in terms of the subjective scores, PSNR, and SSIM. The triangle (mid) function showed the best DMOS performance while the triangle (high) function showed the smallest DMOS improvement. In terms of PSNR and SSIM, the triangle (mid and low) functions showed slightly improved performance for high bitrates.

Fig. 21

Bitrate and subjective score comparison for the frequency quantization set 3 (VGA): (a) triangle (low), (b) triangle (mid), and (c) triangle (high).

Fig. 22

Performance comparison for the frequency quantization set 3 (VGA): (a) subjective score, (b) PSNR, and (c) SSIM.

Figure 23 shows the experimental results of the bitrates and the subjective quality of the frequency quantization set 4 with VGA source sequences. In the frequency quantization set 4, the triangle, cosine, quadratic, and trapezoid functions are used. These functions have similar strategies with different shapes, large quantization steps for middle frequencies, and small quantization steps for low and high frequencies. The bitrate reductions were proportional to the areas of quantization functions. Figure 24 shows performance comparison in terms of DMOS, PSNR, and SSIM (VGA, set 4). Table 4 shows coding efficiency comparisons among the four different functions. In these subjective experiments, the trapezoid function showed the best coding efficiency while the cosine function showed the worst coding efficiency for subjective quality.

Fig. 23

Bitrate and subjective score comparison for the frequency quantization set 4 (VGA): (a) triangle, (b) cosine, (c) quadratic, and (d) trapezoid.

Fig. 24

Performance comparisons for the frequency quantization set 4 (VGA): (a) subjective score, (b) PSNR, and (c) SSIM.

Table 5 shows the processing time comparison of the reference model (H.264) and the proposed models (3.40 GHz Intel i7-3770 CPU, 8 GB Memory). Since the proposed methods used frequency shaping functions, they did not increase the processing time. Also, since the quantization matrix is already included in the H.264/AVC standard, the proposed method can be easily implemented.

Table 5

Processing time comparisons.

6.

Conclusions

In this paper, we have investigated the frequency sensitivity of the human visual system as applied to video compression standards, especially the H.264/AVC standard. Most conventional standards for video compression use the DCT method. On the other hand, those standards do not always consider the frequency sensitivity of the human visual system. In our experiments, subjective quality assessments for video quality were performed to provide a better understanding of human frequency sensitivity characteristics. In the future, these frequency characteristics may be used to improve video coding efficiency while maintaining equivalent perceptual video quality.